Journal of Applied Sciences1812-56541812-5662Asian Network for Scientific Information10.3923/jas.2007.66.71AbaciKadir UyarogluYilmaz YalcinM. Ali YildizMurat 1200771Power transfer margins are studied with Under Load Tap Changing (ULTC) transformers based on both static and dynamic analysis. The P-V Curve, Q-V Curve, have been widely used to analyze power system behaviors under varying loading conditions. Voltage stability analysis and loadability analysis are examples of the application of these curves in power system analysis. Voltage stability of a power system is related to load characteristics and voltage control devices such as (ULTC) transformers .The regulation of a sample power system’s voltage has adopted automatic ULTC operation in order to maintain a constant voltage level. Dynamic behaviors of major power system components, i.e., different loads, ULTC and mechanically switched shunt capacitor et al., are thoroughly examined by a small yet typical equivalent system. In this paper, the effects of distribution ULTCs are investigated by both static and dynamic analysis. In dynamic analysis, both the ULTC and the load are considered as dynamic devices. The load is modeled as a generic dynamic load model. The ULTC is modeled as continuous dynamic model. Time constants of loads and ULTCs are key factors that affect voltage stability. The effects on system behavior of tap dynamics modeling are illustrated. This study discusses effect of the Under Load Tap Changing transformers (ULTCs) on voltage stability and identifying critical ULTCs to avoid possibilities of voltage instability conditions. This research is intended to decide with simulations results as a first step before tackling under real load case. It is intended as an aid for power systems engineers to get acquainted with the dynamics of voltage collapse for the tap changer. Because of it's simplicity, a detailed mathematical model can easily be derived.]]>Canizares, C.A.,1995Dong, F., Badrul and H. Chowdhury,2004Larsson, M.,2000Popovic, D., I.A. Hiskens and D.J. Hill,1996Roman, C. and W. Rosehart,2006Thukaram, D., L. Jenkins, H.P. Khincha, G. Yesuratnam and B.R. Kumar,2004Van Cutsem, T. and C. Vournas,1998Venkatasubramanian, V.,2000Vournas, C.D.,2002Vu, K.T.,1992Xu, W. and Y. Mansour,1994Zhu, T.X., S.K. Tso and K.L. Ko,2000